Georemediation

Waste products from industrial operations, if not disposed of
properly, have the potential to significantly impact the quality of our
natural surface and ground water systems. Geoenvironmental engineering
focuses on finding engineered solutions to contaminants that have
impacted on groundwater aquifers. This is accomplished through field and
laboratory work which allows both the hydraulic characteristics of the
aquifer and the extent of contamination of the groundwater to be
characterized. This data coupled with pilot scale tests allows the
robustness of existing geochemical transport models to be tested thereby
providing a greater understanding of how the full scale remedial
project will perform over time.

Current Projects

Remediation of inorganic contamination presents
a tremendous challenge because the contaminants are often in elemental
form and so cannot be "destroyed" like organic contaminants can. In
situations where contaminants cannot be naturally degraded or
immobilized external remediation methods such as permeable reactive
barriers may need to be applied. These barriers are simple and effective
passive treatment systems and are very cost effective. The following
diagram is a schematic of a permeable reactive barrier for the
remediation of groundwater contaminated with spent potliner (SPL) waste
from the aluminium smelter industry. Leaching of SPL waste can cause
elevated concentrations of fluoride and cyanide in the local
groundwater.

The aim of this project is to develop technology for the remediation
of inorganic contaminants from groundwater focusing initially on
fluoride & cyanide contamination associated with spent potliner
leachate. This is done via pilot scale testing of various barrier
configurations in the Geoenvironmental contaminant research facility, a
fully isolated, controlled experimental facility.
The key components of the Geoenvironmental Experimental facility are:

Contaminated groundwater storage tanks

Multipurpose contaminant pilot-cell that allows maximum flexibility in the configuration of the barrier set-up and design

A fully automated sampling system to allow continuous monitoring throughout the barrier

Figure 1.4.
The multi-purpose reactive barrier pilot cell contains ten horizontal
and four vertical sections which can be filled with various materials
thereby producing a simulated aquifer in both the vertical and
horizontal directions allowing the recreation of site specific
geological conditions. Small monitoring wells in each section allow real
time chemical analysis of reactive barrier performance.

Figure 1.5b.
Flow through cells at the influent and effluent ends of the pilot cell
allow continuous monitoring of pH, conductivity, and Fluoride, Calcium
and Cyanide concentrations.

Samples collected by the automatic sampler are analyzed using ion
chromatography in the Geoenvironmental Laboratory which contains a
state-ofthe-art Dionex ICS-2500 ion chromatograph which allows analysis
of major cations and anions as well as free and complex cyanides.

The transport, geochemistry and fate of contamination originating
from an Aluminium smelter site in the Hunter Valley, NSW Australia

Since 1969 the Kurri Kurri smelter has continuously used the
Hall-Heroult process to produce molten aluminium from the aluminium ore,
alumina (Al2O3). This refinement process works by placing alumina and
an electrolyte, cryolite (sodium aluminium fluoride, Na3AlF6), into
carbon lined "pots" where an electric current is applied causing the
alumina to dissociate into aluminium and oxygen. The carbon lining of
the "pots" act as the cathode and have a life-span of around 5 years.
When the "pot" fails the carbon cathode is removed from the "pot" shell
and the "pot" is then re-lined with a new carbon cathode. The discarded
pot-liner contains fluoride and sodium from the cryolite solution.
Cyanide is also present and is produced under the extreme temperatures
in the pot from the reaction between carbon from the cathode and
nitrogen from the atmosphere.

Historically, the spent pot liner waste (SPL) was disposed of in open
unlined pits where leachate from the pile was caught in a leachate
collection pond which was surrounded by a bund to collect any overflow.
This began soon after the opening of the plant in 1969 and continued
until 1992 when the environmental implications of the groundwater
contamination around the SPL stockpile was realized. By 1995 the waste
pile was capped thereby minimizing any further leachate production. SPL
waste is now stored in large, onsite sheds which allows the waste to be
isolated from the surrounding environment.

Figure 2.2. Industrial waste pile after capping with two monitoring wells shown in the foreground.

This project aimed to characterize the hydrogeology, geochemistry and
physical extent of SPL leachate contamination on the local aquifer
system and to recommend clean-up procedures for the site. This was
achieved through analysis of groundwater samples taken from an existing
monitoring network and the installation of further monitoring wells to
augment the field data. Cone Penetration Testing (CPT) is also used to
elucidate the subsurface stratigraphy at the site. (Figure 2.3a &
2.3b - click here for Figure 2.3b "CPT results")

Figure 2.3a. The University of Newcastle's CPT test
rig. Cone penetration soundings are an accurate, expedient and efficient
means of geoenvironmental site characterization for delineating site
stratigraphy.

The results obtained from groundwater monitoring were used to produce
contaminant maps defining the extent of the contaminant plume. The
maps, in conjunction with the hydraulic properties of the aquifer were
used to make a 3D surface model and preliminary groundwater flow models
using MODFLOW software to aid in the assessment of the most appropriate
remediation method for the site. Results from corresponding laboratory
and pilot scale tests will enable the most environmentally sound method
of preventing further spreading of SPL contamination to be implemented.

Figure 2.5. Topography of filed site showing surface
water flows to aid in determining spread of groundwater contamination
with most significant contaminated areas shown in fluorescent green.
Vertical axis has been exaggerated.

Figure 2.6. Calcite Permeable Reactive Barrier (PRB)
model results from MODFLOW showing the reduction in fluoride
concentrations transported from under the capped SPL pile at the field
site. The left most figure shows results assuming no
sorption/retardation (Kd =0) as expected, a Kd value of zero produces no
change in the plume. A Kd value of 0.001 (middle figure) mostly
contained the plume, albeit with a fluoride concentration exiting the
PRB of around 100 mg/L. A Kd value of 0.01 (right figure) was predicted
to be adequate for the PRB scenario described above.

Note: the model process used was an extreme simplification using
linear isotherms (most suited for sorption behaviour at low contaminant
concentrations). At high pH (>9), fluoride sorption onto calcite
should be modelled as a Langmuir isotherm. The calcite Kd value is
0.007198 L/g at pH 10.0 as determined in laboratory studies. Calcite PRB
installation shown as double line.